A mobile radio terminal has been recently widespread. Furthermore, a demand for fast data services of the mobile radio terminal is expanding. Along with these recent situations, it has become much more important to increase the transmission output power of a base station. A high power amplifier (HPA) is used for increasing to increase the transmission output power of a base station. Moreover, the high power amplifier requires high power efficiency in addition to the increase of transmission output power. Therefore, a composite high power amplifier (C-HPA) having a plurality of HPAs has been introduced to amplify transmission output power and further raise power efficiency. When a base station employs C-HPA, there is also an advantage that input signals can be uniformly amplified. The C-HPA includes, for example, an amplifier using an outphasing method and a Doherty amplifier. An outphasing method means an amplification method for performing linear amplification by using nonlinear components that is called LINC (Linear Amplification with Nonlinear Components). A Doherty amplifier means an amplifier that simultaneously activates two amplifiers during high power operation and activates only one amplifier during low power operation.
In order to achieve high power efficiency, these high power amplifiers are often driven into a nonlinear region that causes intermodulation distortion. Nonlinearities in the HPAs distort signals, thereby causing EVM (Error Vector Magnitude) and BER (Bit Error Rate) degradations. Therefore, various distortion compensation techniques have been proposed in order to satisfy both of power efficiency and linearity of a high power amplifier. A digital predistortion (DPD) technique is one of distortion compensation techniques that could lead to more efficient and cost-effective high power amplifiers.
FIG. 10 is a diagram explaining a conventional example of a DPD-type C-HPA. In FIG. 10, the C-HPA includes amplifiers 301 to 303, tap couplers 311 to 313, a coupler 320, subtracters 331 to 333, LMS (Least Mean Square) processors 341 to 343, multipliers 351 to 353, and an antenna 360. Hereinafter, when the amplifiers 301 to 303 are not distinguished, they are simply referred to as “amplifiers 300”. When the tap couplers 311 to 313 are not distinguished, they are simply referred to as “tap couplers 310”. When the subtracters 331 to 333 are not distinguished, they are simply referred to as “subtracters 330”. When the LMS processors 341 to 343 are not distinguished, they are simply referred to as “LMS processors 340”. When the multipliers 351 to 353 are not distinguished, they are simply referred to as “multipliers 350”. Although only the three amplifiers 300 are illustrated in the present embodiment, the C-HPA of FIG. 10 may actually have the N amplifiers 300. The amplifier 303 corresponds to the N-th amplifier. In this case, the number of the tap couplers 310 corresponding to the individual amplifiers 300 is “N”. Similarly, the number of the subtracters 330 is “N”, the number of the LMS processors 340 is “N”, and the number of the multipliers 350 is “N”.
As illustrated in FIG. 10, signals y*i (i=1 to N) output from of the individual amplifiers 300 are input into the tap couplers 310. The individual tap couplers 310 return the signals y*i into signal levels before amplification and output the returned signals to the subtracters 330 as feedback signals y*i. Then, the individual subtracters 330 subtract the feedback signals y*i from input signals xi (i=1 to N) to obtain error signals. Then, the individual subtracters 330 output the obtained error signals to the LMS processors 340. The individual LMS processors 340 perform an LMS process on the error signals input from the subtracters 330 to compute correction (predistortion) signals hi (i=1 to N). Then, the individual LMS processors 340 output the correction signals hi to the multipliers 350. The individual multipliers 350 multiply the input signals xi by the correction signals hi. In this case, because each of the signals xi is a complex I/Q signal of which the real part is an I-signal and the imaginary part is a Q-signal, each of the multipliers 350 actually performs complex multiplication. In this manner, inverse characteristics of distortion characteristics of the individual amplifiers 300 are added to the input signals xi by multiplying the input signals xi by the correction signals hi. As a result, because distortion caused by each the amplifier 300 is canceled, a signal without distortion is output from each the amplifier 300. The coupler 320 then couples signals without distortion output from the individual amplifiers 300 to generate an output signal. Then, the generated output signal is output from the antenna 360.    Patent literature 1: Japanese Laid-open Patent Publication No. 2003-32055    Patent literature 2: U.S. Pat. No. 6,111,462    Non-Patent literature 1: Qurehi j. H. et al, “90-W Peak Power GaN Outphasing Amplifier With Optimum Input Signal Conditioning”, IEEE Trans On Theory And Techniques, 2009, vol. 57, No. 8, pp. 1925-1935    Non-Patent literature 2: Altera Application Note 314, “Digital predistorter reference design”    Non-Patent literature 3: Ilkka Hakala et al, “A 2.14-GHz Chireix Outphasing Transmitter”, IEEE Trans On Microwave Theory And Techniques, Vol. 53, No. 6, June 2005    Non-Patent literature 4: W. C. Edmund et al, “A Mixed Signal Approach Towards Linear And Efficient N-Way Doherty Amplifiers”, IEEE Trans On Microwave Theory And Techniques, Vol. 55, No. 5, May 2007    Non-Patent literature 5: Paloma Garcla, Jesus de Mingo, Member, IEEE, Antonio Valdovios, and Alfonso Ortega, “An Adaptive Digital Method of Imbalances Cancellation in LINC Transmitters”, IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, Vol. 54, No. 3, MAY 2005    Non-Patent literature 6: P. Jardin and G. Baudoin, “Filter Look Up Table method for Power Amplifier Linearization,” IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, 2007, Vol. 56, MUMB 3, pages 1076-1087
The conventional DPD type C-HPA sends signals output from individual HPAs to subtracters as feedback signals by using tap couplers. In this manner, when the signals output from the individual HPAs are used as feedback signals by using the tap couplers, MMIC (Monolithic Microwave Integrated Circuit) generates excess outputs so as to increase the cost and size and deteriorate the C-HPA reliability. An additional problem with tap couplers implementation is that the tap couplers placed at the outputs of the individual HPAs have insulation and thus they are suffered from the mutual coupling between the outputs of the individual HPAs.
On the contrary, when tap couplers are not used, only a signal that is obtained by coupling the outputs of the individual HPAs can be acquired as the output of the C-HPA. In general, it is difficult to acquire the outputs of the individual HPAs by analyzing the output of the C-HPA and thus it is difficult to find out correction signals hi (i=1 to N) corresponding to transfer functions fi of the individual HPAs.